A method of forming a roofing-grade asphalt mixture can include mixing a bio-asphalt including a partially oxidized bio-source material, a bitumen source material different from the bio-asphalt, and particles to form the roofing-grade asphalt mixture. In an embodiment, the bitumen source material can have a softening point of at least approximately 102° C. and a penetration distance no greater than approximately 20 dmm. In another embodiment, the roofing-grade asphalt mixture can have a softening point of at least approximately 104° C., a penetration distance no greater than approximately 12 dmm, a viscosity of at least approximately 3000 cps at a temperature of 177° C., or any combination thereof. The roofing-grade asphalt mixture can be applied to a base material to form a roofing product.